The present invention relates generally to techniques for imaging objects located in or behind turbid media and more particularly to a novel technique for imaging objects located in, at the surface of or behind turbid media and, additionally, to a novel technique for detecting ice, snow and the like on airplane wings and similar surfaces.
As can readily be appreciated, there are many situations in which the detection of an object present in a turbid, i.e., highly scattering, medium is highly desirable. For instance, the detection of a tumor embedded within a tissue is one such example. One common technique for detecting tumors in tissues uses X-ray radiation. Although X-ray techniques do provide some measure of success in detecting objects located in turbid media, they are not typically well-suited for detecting very small objects, e.g., tumors less than 1 mm in size embedded in tissues, or for detecting objects in thick media. In addition, X-ray radiation can present safety hazards to a person exposed thereto. Ultrasound and magnetic resonance imaging (MRI) offer alternatives to the use of X-rays but have their own drawbacks.
Another technique used to detect objects in turbid media, such as tumors in tissues, is transillumination. In transillumination, visible light is incident on one side of a medium and the light emergent from the opposite side of the medium is used to form an image. Objects embedded in the medium typically absorb the incident light and appear in the image as shadows. Unfortunately, the usefulness of transillumination as a detection technique is severely limited in those instances in which the medium is thick or the object is very small. This is because light scattering within the medium contributes to noise and reduces the intensity of the unscattered light used to form the image shadow.
To improve the detectability of small objects located in a turbid medium using transillumination, many investigators have attempted to selectively use only certain components of the transilluminating light signal. This may be done by exploiting the properties of photon migration through a scattering medium. Photons migrating through a turbid medium have traditionally been categorized into three major signal components: (1) the ballistic (coherent) photons which arrive first by traveling over the shortest, most direct path; (2) the snake (quasi-coherent) photons which arrive within the first .delta.t after the ballistic photons and which deviate, only to a very slight extent, off a straight-line propagation path; and (3) the diffusive (incoherent) photons which experience comparatively more scattering than do ballistic and snake photons and, therefore, deviate more considerably from the straight-line propagation path followed by ballistic and snake photons.
Because it has been believed that ballistic and snake photons contain the least distorted image information and that diffusive photons lose most of the image information, efforts to make transillumination work most effectively with turbid media have focused on techniques which permit the selective detection of ballistic and snake photons while rejecting diffusive photons. This process of selection and rejection has been implemented in various time-gating, space-gating and time/space-gating techniques. Patents, patent applications and publications which disclose certain of these techniques include U.S. Pat. No. 5,140,463, inventors Yoo et al., which issued Aug. 18, 1992; U.S. Pat. No. 5,143,372, inventors Alfano et al., which issued Aug. 25, 1992; U.S. Pat. No. 5,227,912, inventors Ho et al., which issued Jul. 13, 1993; presently-pending and allowed U.S. patent application Ser. No. 07/920,193, inventors Alfano et al., filed Jul. 23, 1992; Alfano et al., "Photons for prompt tumor detection," Physics World, pp. 37-40 (January 1992); Wang et al., "Ballistic 2-D Imaging Through Scattering Walls Using an Ultrafast Optical Kerr Gate," Science, Vol. 253, pp. 769-771 (Aug. 16, 1991); Wang et al., "Kerr-Fourier imaging of hidden objects in thick turbid media," Optics Letters, Vol. 18, No. 3, pp. 241-243 (Feb. 1, 1993); Yoo et al., "Time-resolved coherent and incoherent components of forward light scattering in random media," Optics Letters, Vol. 15, No. 6, pp. 320-322 (Mar. 15, 1990); Chen et al., "Two-dimensional imaging through diffusing media using 150-fs gated electronic holography techniques," Optics Letters, Vol. 16, No. 7, pp. 487-489 (Apr. 1, 1991); Duncan et al., "Time-gated imaging through scattering media using stimulated Raman amplification," Optics Letters, Vol. 16, No. 23, pp. 1868-1870 (Dec. 1, 1991), all of which are incorporated herein by reference.
Of the above-listed art, Wang et al., "Kerr-Fourier imaging of hidden objects in thick turbid media," Optics Letters, Vol. 18, No. 3, pp. 241-243 (Feb. 1, 1993) is illustrative. In this article, there is disclosed a time/space-gating system for use in imaging opaque test bars hidden inside a 5.5 cm-thick 2.5% Intralipid solution. The disclosed system includes three main parts: a laser source, an optical Kerr gate and a detector. The laser source is a picosecond mode-locked laser system, which emits a 1054 nm, 8 ps laser pulse train as the illumination source. The second harmonic of the pulse train, which is generated by transmission through a potassium dihydrate phosphate (KDP) crystal, is used as the gating source. The illumination source is sent through a variable time-delay and is then used to transilluminate, from one side, the turbid medium containing the opaque object. The signal from the turbid medium located at the front focal plane of a lens is collected and transformed to a Kerr cell located at its back focal plane (i.e., the Fourier-transform spectral plane of a 4F system). That portion of the Kerr cell located at the focal point of the 4F system is gated at the appropriate time using the gating source to preferentially pass the ballistic and snake components. The spatial-filtered and temporal-segmented signal is then imaged by a second lens onto a CCD camera.
Although time- and/or space-gating techniques of the type described above have provided a modicum of success in improving transilluminated images, there still remains considerable room for improvement.
It has long been known that the accumulation of ice and/or snow on any lifting or control surface of an aircraft, such as an airplane wing or on a helicopter rotor, can lead to disastrous results. Accordingly, considerable effort has been expended in the past to devise techniques that enable the detection of ice and/or snow on airplane wings and similar surfaces. At present, a variety of ice detection techniques exist which have had varying degrees of success. Some such techniques rely on the thermal detection of ice, others on the electrical or ultrasonic detection of ice. Still other techniques, such as those disclosed in U.S. Pat. No. 5,500,530, inventor Gregoris, which issued Mar. 19, 1996, U.S. Pat. No. 5,484,121, inventors Padawer et al., which issued Jan. 16, 1996, U.S. Pat. No. 5,400,144, inventor Gagnon, which issued Mar. 21, 1995, U.S. Pat. No. 5,296,853, inventors Federow et al., which issued Mar. 22, 1994, and U.S. Pat. No. 5,180,122, inventors Christian et al., which issued Jan. 19, 1993, all of which are incorporated herein by reference, rely on optical or electro-optical detection techinques.
In U.S. Pat. No. 5,475,370, inventor Stern, which issued Dec. 12, 1995, and which is herein incorporated by reference, there is disclosed a system for detecting the presence of an energy polarization altering dielectric material, such as ice or snow, on a surface, such as part of an aircraft, which normally specularly reflects incident energy, such as light, when there is no such dielectric present. The energy is conveyed from a transmitter along a path to the surface and the incident energy is reflected from the surface along a path to a receiver with a dielectric on the surface destroying any polarization, such as circular, of the energy and that reflected from a specular portion maintaining the polarization. An optical system in one or both of the paths operates in an isolator state to produce an image of the dielectric portion having a first intensity level and that of the specular portion passing through the optical system having a different intensity level. When the optical system is operated alternately in isolator and non-isolator states it produces an image of the dielectric portion having a relatively steady intensity level and that of the specular portion alternating between first and second different intensity levels corresponding to the isolator and non-isolator states of the optical system.
One problem noted by the present inventors with the technique of the above-identified Stern patent is that, because the Stern technique is based on the depolarization of specularly reflected light (with ice being treated as a dielectric material that destroys any polarization while metal maintains polarization), appropriate alignment of the plane of reflection of the object must be maintained with respect to the position of the illuminating source and the detector so that the specularly reflected light from the object is directed to the detector.